Marks formed in metal surfaces, such as stamped marks and engraved marks (e.g., laser engraved marks), a prior art example of which is illustrated in FIG. 1 100, provide a means of unique identification for many items including firearms and automobiles. Unfortunately, marks are regularly defaced 110 for criminal activities. However, the marking of the metal surface, e.g., the stamping or engraving process, causes a permanent change in shape of the surrounding metal 102, primarily due to the inability of regions of crystalline arrangement within localized grains to resist the induced stress of stamping, melting, etc., leading to an alteration of the structure and by extension, the interlocking grain boundaries. This resulting deformation extends to some depth below the mark and is known as the zone of plastic strain 104. This area has physical and chemical properties that differ from those of the surrounding non-stamped metal due to the changes in its microstructure.
Many approaches leveraging on these structural features have been developed in an attempt to recover obliterated serial numbers. One widely used approach is chemical etching. This process utilizes the change in chemical potential that makes the defaced area more reactive to acids and thereby allowing for recovery of the defaced number 106 within the zone of plastic strain 104. Chemical etching, however, is a highly controlled and destructive recovery process that requires delicacy and some expertise in applying and removing the etchant. Additionally, because it is a destructive method, the test can only be run once and the test specimen is permanently altered. This shortcoming is the impetus for developing non-destructive, reproducible methods of defaced serial number identification.
A non-destructive approach that has been examined for the detection of defects in metals is infrared thermal imaging. This method seeks to locate and characterize flaws by measuring their effect on heat flow through the material under controlled conditions by observation of the propagation of applied thermal energy. Local regions of plastic strain can be detected as the temperature gradient therein will differ from the rest of the surface due to the local change in thermal conductivity. However, the data acquired through thermographic imaging techniques can be noisy due to undesired signals from several factors including unevenly heated surfaces, radiation from the heated surface and local emissivity variations.
What are needed in the art are methods for recovering defaced marks in metal surfaces. Non-destructive methods that can consistently recover defaced marks in metal surfaces with high confidence would be of great benefit.